For aircraft utilizing turbine engines, a case assembly typically encloses the turbine. Internal to the case assembly, the space surrounding the turbine blades (“the envelope”) may initially be generally circular in cross-section and dimensioned to provide a relatively small gap between the Blade Outer Air Seals (BOAS) that line the envelope of the case assembly and the tip of each rotating turbine blade.
After the engine experiences a break-in period, including some amount of flight time, the gap between the BOAS and the tip of each turbine blade may no longer be consistent due to a variety of reasons. In some portions of the envelope the gap may be greater than in other portions of the envelope. Furthermore, some changes in the gap between the BOAS and the tips of the turbine blades may occur during the various phases of flight due to expansion of the case assembly that surrounds the turbine. Larger than necessary gaps between the BOAS and the tips of the turbine blades may decrease the efficiency of the turbine.